The events of Balin's short-lived lordship in Khazad-dûm were recorded in the Book of Mazarbul. That book later suffered considerable damage at the hands of the Orcs of Moria, and much of its contents became lost or illegible. Among the earlier pages is a passing reference to 'Durin's Axe', and this is clearly an item of some importance, but all the surrounding text has been lost from the book.

This early part of the Book of Mazarbul dates from the time when Balin and his Dwarves were exploring and establishing themselves in Moria, and the most natural reading is that Durin's Axe was some ancient heirloom - presumably the axe of Durin the Deathless himself - that the colonists had recovered. Without any contextual information, though, it's hard to be sure that this is correct: the Dwarves' records might even be registering their disappointment that Durin's Axe could not be found. Whatever the meaning of the words, it is noticeable that no axe of any kind was discovered by the Fellowship as they passed through Moria some thirty years later.

Notes

1

This assumes that the Durin in question was Durin the Deathless, the founder of Khazad-dûm. Conceivably the axe belonged to one of his later namesakes, in which case it could have been made at any time before III 1980 (the date of the death of Durin VI).